The invention relates to a process for improving the echo suppression in a telecommunications system in which undesired electric coupling occurs between a receiving path and transmitting path and gives rise to disturbing echos, so-called line echos. In handsfree telephones and auditorium sound systems, echos, so-called acoustic echos, can also occur due to acoustic coupling between loudspeaker and microphone. In order to be able to successfully carry out an echo suppression, the degree of coupling between transmitting path and receiving path must be defined by a coupling factor CF which indicates the ratio of the signal energy received at the echo reception location to the signal energy transmitted at the echo source. The echo signal y(k) is delayed by the echo delay time i relative to the transmitted signal x(k) so that the coupling factor is                     CF        =                                            ∑                              k                =                0                            N                        ⁢                                          (                                  y                  ⁡                                      (                    k                    )                                                  )                            2                                                          ∑                              k                -                0                            N                        ⁢                                          (                                  x                  ⁡                                      (                                          k                      -                      i                                        )                                                  )                            2                                                          (        E1        )            
wherein
An essential problem in defining the coupling factor CF consists in determining the echo delay time i in Equation (E1) as accurately as possible. To determine the echo delay time i it is necessary to ascertain when the transmitted signal arrives at the echo reception location. As telecommunications systems are generally operated bidirectionally, the echo signal can be superimposed by a signal fed-in as useful signal at the remote end of the transmission path. This is the case whenever speech occurs simultaneously at both ends of the transmission path. This is the so-called double talk situation. In defining the coupling factor CF it is necessary to ensure that the echo delay time i is correctly set and that no double talk is occurring. The clear identification of an echo is a general problem in all possible transmission situations.
To detect an echo it is known to perform a correlation analysis in which the transmitted signal x(k) is recorded in a time interval k=0 . . . N and compared with the signal y(k) incoming at the reception location, see DE-A-42 29 910. The degree of correlation xcfx81 then indicates the probability as to whether an echo is present, xcfx81=1, or a mixed signal is present, xcfx81=0. The decision as to whether the correlation degree xcfx81 is to be interpreted as an echo or as a mixed signal produced by double talk, is made by a threshold value xcex3. The greater the selected threshold value xcex3, the less often are existing echos detected. Thus incorrect decisions are frequently made and the adaptation of the system is blocked. If on the other hand too low a threshold value xcex3 is selected, the danger exists that the system will be adapted in double talk operation and become unstable. The calculation outlay for the correlation analysis, comprising N multiplications, N adaptations and one division, is very high whereas the echo detection reliability is unsatisfactory.
The invention fulfils the objective of providing a process for improving the echo suppression in a telecommunications system which facilitates precise differentiation between echos and double talk, and thus the reliable detection of echos, in particular line echos, whose delay time can amount to up to 640 ms and which can occur up to five times within 64 ms.
Invention, by a process wherein pulse sequences are derived from the characteristic curve of the speech signal as special speech features which are dependent upon the frequency but not the amplitude of the speech signal, so that in the case of a real echo in linear transmission systems, independently of the attenuation of the echo, identical but time-shifted pulse sequences occur in the case of the transmitted signal and in the case of a received echo. The correspondence between the time interval sequences of transmitted signal and received signal is an indication that the received signal is an echo of the transmitted signal. The time shift between the pulse sequences is a measure of the echo delay time i.